In the present day, a photon counting CT device (CT stands for Computed Tomography) is known in which a photon counting type detector is used. Unlike a charge integrating detector, the photon counting type detector is configured to detect individual X-ray photons that have passed through a test subject. Thus, in the photon counting CT device, it is possible to reconstruct X-ray CT images having a high signal-to-noise (S/N) ratio.
Moreover, the photon. counting type detector can be used in measuring (discriminating) the energies of X-ray photons. Hence, in the photon counting CT device, projection data, which is collected. by irradiation of X-rays from an X-ray tube with one type of tube voltage, can be divided into a plurality of energy components and image formation can be performed.
As a detector of the photon counting type, an “indirect-conversion-type detector” is known in which the incident X-ray photons are temporarily converted into visible light (a scintillator light) using a scintillator and then the scintillator light is converted, into electrical signals using an optical sensor, An optical sensor detects individual scintillation photons that are obtained by a scintillator by conversion from radiation, and detects the radiation failing on the scintillator and measures the energy of that radiation. Examples of the optical sensor include a sensor in which a plurality of avalanche photo diodes (APDs), which operate in the Geiger mode, is arranged in an array.
In order to measure the energy of radiation, it is important to accurately measure the number of scintillation photons that are generated. The scintillation photons generated in a scintillator are incident, directly on a photon counting sensor. Alternatively, the scintillation photons repeatedly get reflected from a lateral face or the top face of the scintillator and then are incident on the photon counting sensor. It is often the case that the scintillation photons are generated in the vicinity of the radiation incidence plane of the scintillator. In that case, there is a little distance between the position of generation of the scintillation photons and the position of the photon counting sensor. Hence, the scintillation photons fall on the photon counting sensor in a uniform manner (i.e., fall substantially on the entire photon counting sensor with an appropriate spread).
However, in case the scintillation photons are generated in the vicinity of the interface of the scintillator and the photon counting sensor, a majority of the scintillation photons are incident locally on the photon counting sensor. As a result, it becomes difficult to accurately count the scintillation photons.